Conservative, special-relativistic smoothed particle hydrodynamics

TL;DR

This paper introduces a new special-relativistic SPH formulation that improves shock capturing, conservation, and accuracy in high Lorentz factor scenarios, validated through extensive benchmark tests.

Contribution

It presents a self-consistent relativistic SPH scheme with advanced artificial viscosity and grad-h terms, outperforming previous formulations in accuracy and stability.

Findings

Accurately handles Lorentz factors up to 50,000 in shock tests.

Sharpens shock fronts while reducing post-shock noise.

Performs well in complex relativistic wave and multi-dimensional tests.

Abstract

We present and test a new, special-relativistic formulation of Smoothed Particle Hydrodynamics (SPH). Our approach benefits from several improvements with respect to earlier relativistic SPH formulations. It is self-consistently derived from the Lagrangian of an ideal fluid and accounts for special-relativistic "grad-h terms". In our approach, we evolve the canonical momentum and the canonical energy per baryon and thus circumvent some of the problems that have plagued earlier formulations of relativistic SPH. We further use a much improved artificial viscosity prescription which uses the extreme local eigenvalues of the Euler equations and triggers selectively on a) shocks and b) velocity noise. The shock trigger accurately monitors the relative density slope and uses it to fine-tune the amount of artificial viscosity that is applied. This procedure substantially sharpens shock fronts while still avoiding post-shock noise. If not triggered, the viscosity parameter of each particle decays to zero. None of these viscosity triggers is specific to special relativity, both could also be applied in Newtonian SPH. The performance of the new scheme is explored in a large variety of benchmark tests where it delivers excellent results. Generally, the grad-h terms deliver minor, though worthwhile, improvements. The scheme performs close to perfect in supersonic advection tests, but also in strong relativistic shocks, usually considered a particular challenge for SPH, the method yields convincing results. For example, due to its perfect conservation properties, it is able to handle Lorentz-factors as large as $\gamma= 50 \; 000$ in the so-called wall shock test. Moreover, we find convincing results in a rarely shown, but challenging test that involves so-called relativistic simple waves and also in multi-dimensional shock tube tests.